1. Field of the Invention
The present invention relates to a semiconductor device including a gate insulation film in a layered structure, and a method and an apparatus for manufacturing the same.
2. Description of the Related Art
In recent years, thinning of a gate insulation film has been on progress, with which a reduction in leak current has been required. Hence, trials are made to layer on a silicon oxide film (SiO2 film) a silicon nitride film (SiN film), which has a dielectric constant higher than that of the SiO2 film, and to nitride the surface of a SiO2 film by a remote plasma method to thereby form a SiON film thereon.
In a method for layering the SiN film, dichlorosilane (DCS) or trichlorosilane (TCS) and NH3 are used as a material gas. However, when using this material gas, the SiN film is not uniformly formed on the SiO2 film. Therefore, the surface of the SiO2 film is thermally nitrided at about 700° C. to 850° C. before the formation of the SiN film to thereby form a SiON film thereon. However, when this method is employed to try to obtain a gate insulation film with a thickness of about 1.2 nm, a large amount of nitrogen reaches not only the surface of the SiO2 film but also a silicon substrate during nitridation, so that a threshold value Vth of a field effect transistor varies in the negative direction. Further, there also is a problem that the mobility of holes in a channel decreases.
On the other hand, in the method for nitridation by the remote plasma method, when control is conducted to decrease the nitrogen concentration in the vicinity of the surface of the silicon substrate to avoid the above-described disadvantage, the nitridation of the SiO2 film is not enough, resulting in insufficient effect to reduce the leak current. Besides, when trying to sufficiently reduce the leak current, the problem of the variation in threshold value and the decrease in mobility is not solved as in the above-described layering.
In addition, as a method for forming a nitride film, a method using bis(t-butylamino) silane is also disclosed (Patent Documents 3 and 4).
Prior arts are disclosed in Patent Document 1 (Japanese Patent Application Laid-open No. Hei 7-50347), Patent Document 2 (Translated National Publication of Patent Application No. 2002-523897), the Patent Document 3 (Japanese Patent No. 2962417), and the Patent Document 4 (Japanese Patent No. 3055123).